1. Field of the Invention
The present invention relates to a secure analog signal communication system, and more particularly to a Fourier masking analog signal communication system which preserves the bandwidth of the original signal by performing the masking operation in the frequency domain.
2. Description of the Prior Art
In order to provide privacy in a communication system, apparatus is used that renders an analog communication signal unintelligible by altering or "scrambling" the signal in a prearranged way. The intended receiving party uses apparatus to descramble the signal and recover the transmitted information easily while any unintended receiving party experiences considerable difficulty in doing so. Such apparatus finds utility in the field of military, police or other official communications and in the field of civilian communications such as provided by the domestic telephone system. Throughout the following description, the analog communication signal is assumed to be speech, and the communication channel is assumed to be a telephone channel, although it will be understood that wider application of these techniques is envisioned and may include virtually any analog signal and any communication channel having limited bandwidth.
Speech scrambling is provided in the prior art in two basically dissimilar ways, digital scrambling and analog scrambling, where digital scrambling has the potential for providing a greater degree of security than analog scrambling. An exemplary digital scrambling system is disclosed in U.S. Pat. No. 4,052,565 issued to D. D. Baxter et al on Oct. 4, 1977, which relates to a digital speech scrambler system for the transmission of scrambled speech over a narrow bandwidth by sequence limiting the analog speech in a low-pass sequence filter and thereafter multiplying the sequence limited speech with periodically cycling sets of Walsh functions at the transmitter. At the receiver, the Walsh scrambled speech is unscrambled by multiplying it with the same Walsh functions previously used to scramble the speech. The unscrambling Walsh functions are synchronized to the received scrambled signal so that, at the receiver multiplier, the unscrambling Walsh signal is identical to and in phase with the Walsh function which multiplied the speech signal at the transmitter multiplier.
There is, however, a substantial increase in bandwidth of a digital scrambling system as disclosed hereinabove, which is especially disadvantageous when employed in a practical transmission system such as a telephone system. For example, a sampling rate of 8000 samples per second is suitable for a 3.5 KHz speech signal, where for eight-bit samples this rate results in a potential scrambled signal bit rate of 65 Kbps. Therefore, for transmission over a telephone channel this scrambling signal bit rate will require a bandwidth considerably in excess of 3.5 KHz. Alternatively, techniques may be employed to reduce the required bandwidth to 3.5 KHz, but these techniques introduce unwanted distortion and will result in a loss of fidelity.
In contrast, analog scrambling is limited in bandwidth to the bandwidth of the original signal. Thus, a 3.5 KHz telephone speech signal will occupy approximately 3.5 KHz in scrambled form and can be transmitted over ordinary telephone lines without the necessity for additional bandlimiting of the scrambled signal. One known technique for achieving analog scrambling of speech signals is disclosed in U.S. Pat. No. 4,126,761 issued to D. Graupe et al on Nov. 21, 1978. As disclosed therein, an input audio frequency analog signal, as for example, speech, which is to be passed through a noisy transmission channel, is scrambled at the sending end by repetitively performing a modulo-.nu. (MOD .nu.) addition of an n-level, m-pulse codeword with an n-level digitized transformation of the input signal under the condition that m and .nu. are integers. Descrambling is achieved by carrying out a MOD .nu. subtraction process involving repetitively subtracting the same code word from an n-level digitized transformation of the received signal, the subtraction being carried out in synchronism with the addition at the sending end. The resultant difference signal is a representation of the input signal and is relatively insensitive to noise present in the transmission channel. Synchronization is achieved by providing for the codeword to be shifted, at the receiving end, forwardly or backwardly, by an appropriate number of discrete intervals until intelligibility is achieved. Thus, synchronization is achieved by relying on the contents of the received signal.
The disadvantage of analog scrambling, however, is the limited security offered. Because of the complexity and precision required by the circuitry employed, a determined interceptor may find it straightforward to descramble the intercepted signal by exhaustively trying all possible combinations of scrambling variables.
It has, therefore, been a problem in the prior art to provide a scrambling system that has the advantage of the high security afforded by digital scrambling without expanding the bandwidth of the scrambled signal and thus either requiring a broadband communication channel or inducing distortion and loss of fidelity. Restated, the problem is to provide a secure analog speech scrambling system.